This chapter will discuss more about methodology of this project has been outlined to achieved the desired output. This methodology divide into five basic parts which are research, unbalanced load problem, time delay Current Controller Design, Gate Driver circuit design, current sensor design, Active Power Filter Design and lastly interfacing between raspberry PI and hardware.
3.1 Block Diagram NON LINEAR LOAD 3 PHASE INVERTER GATE DRIVER CIRCUIT TIME DELAY CONTROLLER CURRENT SENSOR
Figure 3.1: Block diagram of the project
Figure 3.1 shows the block diagram of the whole project of development of time delay current controller using raspberry pi, This development of Active Power Filter (APF)
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consists of five main parts which are the DC power supply as the input, three phase inverter, current sensor as a feedback input current to the controller, gate driver circuit as an isolator circuit and PWM signal generator, and time delay current controller. The first part is when a DC input voltage fed into a three phase inverter system, this will transform the inverter and performs it as Active Power Filter (APF). The gate drivers use a 5 Vdc while the inverter input voltage depends on the loads that need to be powered.
Second part is the gate driver circuit which has a function that to double up the PWM signal from the Raspberry Pi and also perform as isolator circuit. The gate driver will double up the number of the amplitude of the signal PWM to the three phase inverter. For this project, the gate driver will produce six PWM signal from controller and it means every gate of power MOSFET or IGBT will control by Raspberry Pi.
Third part is 3 phase inverter, the general function of the inverter is to convert the DC voltage to the AC voltage. The focus of three phase inverter in the system is to produce a quality of three phase current APF to the system. The 3 phase inverter also known as the six switch inverter type or full bridge inverter and it will receive the output PWM from the gate driver. The fourth part is current sensor, Current sensor are connected to each voltage phases for measure the line current that flow from supply to nonlinear load and the output of sensor will used as a feedback controller.
The main part of the APF system is the controller. The time delay current control methods will act as the controller unit that embedded into Raspberry Pi. For the Raspberry Pi to perform it purpose it will need an analogue to digital converter to change the input signal which are in continuous waveform into digital signal. Then the controller will subtract the line current with the reference current and the result will feed into DSP controller that is Proportional Integral (PI) and Integral to digitalize the result and compare with feedback current to generate the pulse width modulation using S-R flip-flop with clock to feed into the three phase inverter which are embedded into the Raspberry Pi. With feeding the PWM signal to the inverter, it will rescale the current output to power up the load. For the load component consist of six diode that are connected to the three phase power supply to act as nonlinear load. The diode will interrupt the fundamental waveform of current from supply and create “double hump” waveform.
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3.2 Gate Driver Design
The gate driver circuit are design to amplify the PWM input from the current controller, it is because of the pulse and amplitude doesn’t meet the criteria for the power transistor for switching purpose and isolate controller circuit with power circuit. The circuit are consist of several component that are listed in Table 3.1.
Table 3.1: List of the components for gate driver circuit
No Component Unit 1 IC 7414 3 2 IC 4081 3 3 HCPL3120 6 4 Capacitor 1nF 6 5 Resistor 4.3kΩ 2 6 Resistor 560Ω 2 7 Resistor 10Ω 2 8 Resistor 10kΩ 12
Figure 3.2 shows the schematic diagram of gate driver circuit that will be design. The signal PWM that will be generate by the raspberry pi controller will split into two PWM signal by the Hex Schmit-Trigger Inverter or NOT gate (IC 7414) and it will flow through the zener diode as a protection to the circuit from current feedback. Inverter circuit need 2 signal that not switch ON at the same time, so the first output of the IC 7414 will remain same with the output of the controller but the second output are invert from the first output
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Figure 3.2: The schematic diagram of the gate driver circuit
Both of the output from the NOT gate IC 7414 flow into the Quad 2 input AND gate IC 4081. The IC 7414 consists of four AND gate. Two output signals from the IC 7414 were add to each other to double up the amplitude of the PWM before flow into the Gate Drive Optocoupler (HCPL3120).
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Figure 3.3: Hardware of gate driver circuit
Optocoupler HCPL3120 consists of LED optically that are coupled to an integrated circuit with a power output stage; it works like an isolator circuit from controller circuit that generate PWM to the switching component (power IGBT or MOSFET) and it is ideally suited for driving power IGBT’s or power MOSFET’s used in a nonlinear load inverter application. The minimum supply voltage of this Optocoupler is 15V that are taken directly from supply.
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3.3 Time Delay Current Controller Design
Figure 3.4: Design of the time delay current controller.
Although the current controller provides a very fast dynamic response, it is very sensitive to the time delay in digital control systems. In a conventional digital control system the time delay is usually at least one sampling step because of data sampling, computation and PWM control signal sending out to process, so it is only at the next sampling instant that the consequence of this control action will be observed.
The idea is to create a system that can be controlled as if it was its own minimum phase equivalent. Some knowledge about the controlled system is required to be able to predict its reaction. With this information, it is possible to make the predictable current signal available to the feedback loop before the delay time. This is done by adding the predicted current to the feedback signal without a delay. The feedback loop then “sees” the predicted value of the current at the same time, when it is actually generated. In digital motion control systems this is easily implemented. The predicted signal has to be subtracted from the feedback signal again after the delay time has elapsed, because at that time the regular feedback signal contains the information of the real effect of the voltage command.
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